(1) Field of the Invention
The present invention relates to a cassette for a printing ribbon for typewriters comprising a casing accommodating a supply spool on which the printing ribbon is wound, a receiving spool on to which the ribbon is re-wound after having been used, and a brake and take up device which operates on a tube of the supply spool and on the ribbon which is unwound from said supply spool.
(2) Description of the Prior Art
Published European Patent Application No. 0 075 084 discloses a cassette of this type, in which the brake-take up device comprises an omega-shaped spring which is applied against the external surface of an extension portion of the tube of the supply spool. The omega-shaped spring comprises two arms of which a first arm is fixed to the casing and the second arm has a L-shaped bent portion which is engaged to guide and tension the ribbon as it is unwound from the supply spool. The tension of the spring holds the omega-shaped part bearing against the external surface of the tube and the supply spool is braked. As the ribbon is advanced, the tension of the ribbon causes the second arm of the spring to flex, thereby expanding the omega-shaped part thereof. The latter no longer bears against the external surface of the tube and the supply spool is free to rotate. The second arm of the spring oscillates continuously, tensioning the ribbon which is unwound from the spool and alternating braking actions with release actions.
That device suffers from disadvantages. In fact, in order for the omega-shaped spring to operate effectively, the extension portion of the tube must be of suitable size and thus the cassette will have to be thicker than is necessary, with the result that it is bulky and cumbersome. In addition, the spring being formed by piano wire, engagement of the omega-shaped portion with the extension portion of the tube, after a brief period of operation, gives rise to play between the components. The braking effect decreases whereby there is a reduction in the control over the rotational movement of the supply spool with a lower degree of tensioning of the ribbon.
U.S. Pat. No. 4,505,605 also discloses a cassette for a printing ribbon in which the brake-take up device comprises two springs. A first spring has one end bent at an angle of 90.degree. and introduced into a central hole in a supply spool. The tension of the ribbon holds the external part of the ribbon which is wound on the supply spool constantly pressed against a wall of the container to apply friction to the ribbon and keep it at a predetermined tension. For that purpose a part of the wall has a series of projections or ribs projecting from the wall by variable amounts such as to compensate for the variations in tension of the spring due to the reduction in diameter of the supply spool. A second spring is engaged with the ribbon which is unwound from the supply spool, to perform the function of providing a take-up effect and tensioning of the ribbon. That brake-take up device also suffers from disadvantages. Indeed, during printing cycles in which the ribbon is advanced, the rubbing effect on the ribbon which is pressed against the projections or ribs due to the action of the first spring causes wear at the outside surface of the ribbon, with the risk of losing pigments. In addition, with the ribbon being of greatly reduced thickness, it undergoes deformation to such an extent as to have parts which become limp, causing a deterioration in the quality of printing.